The present invention applies in particular to tight and locked assemblies of two cast iron pipes.
Document EP-A-526 373 describes a composite seal comprising a sealing body and an anchoring heel made of resilient material and a plurality of locking inserts made of rigid material embedded in the anchoring heel. These inserts are distributed uniformly over the whole circumference of the body. Each insert is produced from metal and has an anchoring head designed to rest on the bottom of an anchoring groove arranged in a socket end, and also catching teeth adapted to engage in the outer surface of a spigot so as to prevent axial movements that might separate the spigot from the socket end under the action of axial forces caused by the pressure of the fluid circulating through the spigot and the socket end.
The inclination of each locking insert depends on the play between the external diameter of the spigot and the internal diameter of the socket end. Catching of the insert on the spigot induces a reaction force, the angle of inclination of which over the median direction varies depending on the play present between the assembled ends.
The greater this angle of reaction, measured in relation to the radial direction, the more the locking resists the internal pressure of the fluid circulating through the assembly. On the other hand, catching of the inserts in the outer surface of the spigot improves as the angle of reaction gets smaller. Indeed, if the angle is too great, there is a risk that the teeth of the inserts may not catch on the spigot when it is put under pressure, and may slide thereon causing faulty locking.
The risk of poor performance under pressure is critical with regard to the maximum amounts of play where the angle of reaction is naturally small, while the risk of poor catching of the inserts is critical with regard to the minimum amounts of play where the angle of reaction is naturally high.